A post-Brexit transition is “not a given”, the European Union’s Brexit negotiator Michel Barnier warned Britain on Friday, saying London had raised “substantial” issues with the adaptation period as proposed by the bloc. Barnier, speaking to journalists in Brussels after the latest round of negotiations with British envoys, said he was surprised by London’s position and that some of the EU’s lines on the transition period were non-negotiable.

“If these differences persist, a transition is not a given,” Barnier said. “If these disagreements were to persist, there will undoubtedly be a problem. I hope we will be able to resolve these disagreements in the next round.”

Sterling weakened against the euro as Barnier listed London’s issues with the post-Brexit transition offered by the EU until the end of 2020.

He said Britain rejected giving lifetime rights to EU citizens who arrive after Brexit but before 2021, demanded mechanisms to be able to avoid any new EU laws it disliked, and lashing out against a mechanism the EU wants to be able to unilaterally suspend Britain’s access to the single market in case of disputes.

“The UK insisted that we should reach an agreement in March on this transitional period. At the same time, however, our partners set out a certain number of disagreements, which I regard as substantial,” Barnier said.

He said the EU was still waiting for London to explain what sort of future relationship it wants with the EU and how to avoid an Irish border if, as Prime Minister Theresa May had said, it were to leave the bloc’s customs union in the future.

“The sooner the UK makes its choices, the better,” Barnier also said. “A UK decision to leave the single market and leave the customs union would make border checks unavoidable.”

“We focus on solutions to avoid hard border… any solution must be precise, clear and unambiguous,” Barnier said. “We are waiting for such a solution.”

Barnier also made clear Britain had to sort out all the outstanding issues related to its exit from the bloc – including ties with Euratom and privacy laws, among others – in order to be given a transition period after it leaves the EU, which is now due to happen next March.

“No transition will be possible if there is no withdrawal agreement,” he said after many EU diplomats and officials have warned in recent days over delays putting under threat the whole Brexit schedule that EU and Britain agreed on to give some certainty to the businesses and people affected.

In the status quo transition the EU offered to Britain – envisaging that London remains bound by all EU laws and pays into the bloc’s budget but has no say on any decisions of the union – Britain’s Brexit negotiator particularly disliked what was described by some as a single market “punishment” clause.

Davis said the EU’s approach was “discourteous”, to which Barnier responded on Friday by saying: “I don’t really understand why this reaction, this uproar, we do not wish to punish whatsoever.”

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